1. Field of the Disclosure
The present disclosure relates to an organic electroluminescent display device, and more particularly, to an organic electroluminescent display device and a driving method of the same.
2. Discussion of the Related Art
Organic electroluminescent display (OELD) devices have been proposed and developed to solve problems of liquid crystal display (LCD) devices that are not self-luminous. The OELD devices are self-luminous display devices, which emit light by electrically exciting fluorescent organic compounds. The OELD devices can be driven by low voltages and can have relatively a thin thickness. OELD devices including thin film transistors as a switching element in each pixel are be referred to as active matrix OELD (AMOELD) devices.
FIG. 1 is a view of a pixel structure of an organic electroluminescent display device according to a first embodiment of the related art, and FIG. 1 shows a pixel including two transistors and one capacitor.
In FIG. 1, the pixel includes a switching transistor SW, a capacitor C, a driving transistor DR and an organic light-emitting diode OLED. The switching transistor SW and the driving transistor DR are thin film transistors including amorphous silicon (a-Si:H) and are NMOS (n-channel metal-oxide-semiconductor) transistors.
A gate electrode of the switching transistor SW is connected to a scan line S, and a source electrode of the switching transistor SW is connected to a data line D. One electrode of the capacitor C is connected to a drain electrode of the switching transistor SW, and the other electrode of the capacitor C is connected to a base voltage VSS, which may be ground potential. A gate electrode of the driving transistor DR is connected to the drain electrode of the switching transistor SW and the one electrode of the capacitor C, a source electrode of the driving transistor DR is connected to the base voltage VSS, and a drain electrode of the driving transistor DR is connected to a cathode electrode of the organic light-emitting diode OLED. An anode electrode of the organic light-emitting diode OLED is connected to a power supply line VDD providing driving voltages.
A driving method of the organic electroluminescent display device having the pixel structure of FIG. 1 will be explained with reference to FIG. 2. FIG. 2 shows a timing chart of the organic electroluminescent display device of FIG. 1.
The switching transistor SW turns ON by a positive selection voltage VGH, which is supplied to an nth scan line S(n) (n is a natural number) from a gate driving integrated circuit (not shown), and the capacitor C is charged due to a data voltage Vdata supplied to the data line D. The data voltage Vdata is positive because the driving transistor DR has an n-type channel. Intensity of currents flowing through the channel of the driving transistor DR depends on potential difference between the data voltage Vdata stored in the capacitor C and the driving voltage VDD, and the organic light-emitting diode OLED emits light according to the intensity of the currents.
In the two-transistor and one-capacitor pixel structure, to continuously keep the driving transistor DR on after applying the positive data voltage Vdata, the driving transistor DR including amorphous silicon (a-Si:H) receives the positive voltage stored in the capacitor C. This further increases deterioration of the driving transistor DR and causes changes in a threshold voltage and mobility of the driving transistor DR. Accordingly, currents are not stably provided to the organic light-emitting diode OLED, and quality of displayed images are lowered.
To solve the problem, another pixel structure has been suggested.
FIG. 3 is a view of a pixel structure of an organic electroluminescent display device according to a second embodiment of the related art, and FIG. 4 is a timing chart of the organic electroluminescent display device of FIG. 3. FIG. 3 shows a pixel including four transistors and two capacitors, and the pixel of FIG. 3 includes two portions symmetrical to each other, each of which has a two-transistor and one-capacitor (2T-1C) structure of FIG. 1. The transistors of FIG. 3 are NMOS transistors.
Degradation is compensated by applying a negative voltage to a driving transistor of one 2T-1C portion during a driving timing of the other 2T-1C portion, and compensating degradation is alternately performed at each frame.
Referring to FIG. 3 and FIG. 4, one scan timing 1 ST is divided into two parts, and a first scan signal Vg1 and a second scan signal Vg2 are sequentially applied to a first scan line S1 and a second scan line S2.
In an even frame, a data voltage Vdata having a normal level is applied to the pixel through a first switching transistor SW1 and a first driving transistor DR1 during a timing of applying the first scan signal Vg1, and then a data voltage Vdata having a negative voltage value is applied through a second switching transistor SW2 during timings t1 and t2 of applying the second scan signal Vg2, thereby compensating degradation of a second driving transistor DR2.
Similarly, in an odd frame, a data voltage Vdata having a normal level is applied the pixel through the second switching transistor SW2 and the second driving transistor DR2 during a timing of applying the second scan signal Vg2, and then a data voltage Vdata having a negative voltage value is applied through the first switching transistor SW1 during timings t3 and t4 of applying the first scan signal Vg1, thereby compensating degradation of the first driving transistor DR1.
However, the second embodiment of the related art, which alternately compensates degradation of the first and second driving transistors DR1 and DR2 at each frame, requires more transistors and capacitors than the first embodiment of the related art. In addition, the number of scan lines also increases. Moreover, the driving speed should be at least two times faster than the first embodiment of the related art or the number of gate driving ICs should be increased because one scan timing 1ST of FIG. 4 is divided into two parts and two scan signals are applied.